The invention relates to a method for admission control in an interference-limited cellular radio network, the cellular radio network comprising a network part, at least one subscriber terminal and a bidirectional radio connection between the network part and the subscriber terminal, and the radio connection being at least on one channel, and in the network part are performed
determination of the interference threshold of the channel,
measurement of the interference level of at least one free channel,
comparison of the measured interference level of the free channel with the interference threshold, and decision whether the channel is usable or unusable.
In a cellular radio network information is transferred between a network part and a subscriber terminal by transmitting signals. Five different factors may distort the signal on its way from a transmitter to a receiver: modulator of a transmitter, means of transmission, i.e. radio waves, interference sources, fading, and demodulator of a receiver. The whole formed by these factors is called a channel. Each base station has a plurality of channels at its disposal. The channels may be separated from one another by means of time (TDMA), frequency (FDMA), codes (CDMA) or any combinations of these methods. So on the channel information is transferred in a signal form. The present invention relates to interference sources. The interference sources particularly refer to interference of adjacent channels and interference caused by remote transmitters operating on the same channel.
One purpose of frequency planning is to reduce interference that transmitters operating on the same channel cause to one another. By frequency planning it is determined which frequencies can be used in which cells. To keep the interference on the channel sufficiently low, the transmitters operating on the same (or on the adjacent) channel are to be sufficiently far apart. This is known as a frequency reuse. The distance between transmitterreceiver pairs operating at the same frequency has to be of a certain length, for instance three times the cell diameter. Various reuse patterns have been designed, for instance, the seven-cell reuse pattern. In accordance with these kinds of solutions a new radio connection can always be established, if there is at least one free channel, since the transmitters operating on the same or adjacent channel are so far apart that the interference level is always sufficiently low.
Since the number of cellular radio network users has increased considerably during the past few years, the same frequencies must be reused in cells that are very close to one another. This results from the fact that only a very restricted frequency band is allocated to each network operator. The frequency band has to be utilized efficiently, otherwise the network cannot simultaneously hold enough users, and when a new user is trying to establish a connection, it cannot be admitted, the connection is blocked, i.e. no radio connection is allocated to the user.
A cellular radio network where the same frequencies are reused in cells so close to one another that the connection quality is limited by interference caused by other users, not by background noise, is called interference limited. If interference on the channel is intense, a radio connection can not necessarily be allocated to a new user, since consequently, speech or other payload cannot be transferred reliably, even though the channel itself were free. This has notable drawbacks, since if the channel is allocated for use, the user finds the quality of service provided poor and may even destroy connections of other users. Consequently, some other criterion than just the freedom of the channel has to be found for a connection to be established or to be blocked.
The aim of the present invention is to provide a method by which the admission control can be implemented effectively, and which simultaneously enables an effective frequency reuse.
This is achieved with a method set forth in the preamble, the method being characterized in that it comprises the steps of
assigning the channel number threshold,
deciding whether the number of free channels below the interference threshold exceeds the channel number threshold, when at least one free channel below the interference threshold is selected and allocated to a new radio connection, or whether the number of free channels below the interference threshold goes below the channel number threshold, when a new radio connection is blocked.
The invention further relates to an interference-limited cellular radio network which comprises a network part, at least one subscriber terminal and a bidirectional radio connection between the network part and the subscriber terminal, and the network part is arranged
to define the interference threshold of the channel,
to measure the interference level of at least one free channel,
to compare the measured interference level of the free channel with the interference threshold of the channel, and to decide whether the channel is usable or unusable.
The system is characterized in that the network part is arranged
to assign the channel number threshold,
to decide whether the number of free channels below the interference threshold exceeds the channel number threshold, when at least one free channel below the interference threshold is selected and allocated to a new radio connection, or whether the number of free channels below the interference threshold goes below the channel number threshold, when a new radio connection is blocked.
Several advantages are achieved with the method of the invention. It allows efficient definition whether new users can be admitted to the network or not. If the usability of the channel depends on the dynamically varying interference level, so the described method both enables the efficient use of radio resources and guarantees an acceptable quality to the user. The method of the invention advantageously realizable, and it is technically simple and efficient.
The system according to the invention has the same advantages as those described above in relation to the method. It is obvious that preferred embodiments and detailed embodiments can be combined with one another to provide various combinations in order to achieve the desired technical efficiency.